Since the end of the 20th century the cellular telephone industry has had enormous development in the world. From the initial analog systems, such as those defined by the standards AMPS (Advanced Mobile Phone System) and NMT (Nordic Mobile Telephone), the development has during recent years been almost exclusively focused on standards for digital solutions for cellular radio network systems, such as D-AMPS (e.g:, as specified in EIA/TIA-IS-54-B and IS-136) and GSM (Global System for Mobile Communications). Different digital transmission schemes are used in different systems, e.g. time division multiple access (TDMA) or code division multiple access (CDMA). Currently, the cellular technology is entering the so called 3rd generation, providing several advantages over the former, 2nd generation, digital systems referred to above. Among those advantages an increased bandwidth will be provided, allowing effective communication of more complex data. The 3rd generation of mobile systems have been referred to as the UMTS (Universal Mobile Telephony System) in Europe and CDMA2000 in the USA, and is already implemented in Japan to some extent. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure.
A lot of effort has been made in making smaller terminals, with much help from the miniaturisation of electronic components and the development of more efficient batteries. In only a couple of decades the communication systems have gone from analogue to digital, and at the same time the dimensions of the communication terminals have gone from briefcase size to the pocket size phones of today. Today, numerous manufacturers offer pocket-sized terminals with a wide variety of capabilities and services, such as packet-oriented transmission and multiple radio band coverage. In order to reduce the size of the portable radio terminals, built-in antennas have been implemented over the last couple of years. The general desire today is to have an antenna, which is not visible to the customer. Today different kinds of patches are used, with or without parasitic elements. The most common built-in antennas currently in use in mobile phones include are the so called planar inverted-F antennas (PIFA). This name has been adopted du to the fact that the antenna looks like the letter F tilted 90 degrees in profile. Such an antenna needs a feeding point as well as a ground connection. If one or several parasitic elements are included nearby, they can be either grounded or dielectrically separated from ground.
The development in electronics has not only made it possible to miniaturise the components of the terminals, the complexity and variety of advanced functions and services which the terminals are capable of performing is also ever increasing. The development of new transmission schemes, the 3rd generation mobile system standing at the door and the 4th generation to be expected maybe ten years later, provides the possibility to convey more advanced data to the wireless communication terminals, such as real time video. In order to provide good transmission and reception performance in a multi pass environment, a diversity antenna system or MIMO (Multiple Input Multiple Output) antenna system is required. This will for instance be important for WLAN (wireless local area network) and 3G and 4G cellular mobile terminals.
For circularly-polarised radio waves, a dominant-mode patch antenna is often used as a flat antenna. An antenna of this structure comprises a ceramic substrate and a patch antenna element provided on the surface of the ceramic substrate. Further, a ground conductor provided on the side of the ceramic substrate opposite to the side where the patch antenna element is disposed. A feeding pin is connected to a feeding section provided on the reverse side of the patch antenna element, by way of a through hole formed in the ceramic substrate and through the ground conductor. In principle, in the dominant-mode patch antenna, two sides which are orthogonal to each other within a plane, must be formed to an electrical length of substantially ½ wavelength. In order to make the dominant-mode patch antenna 5 compact, a dielectric substrate having a large dielectric constant must be used as the dielectric substrate. For example, the length of the side of the antenna in a GPS vehicle-mounted receiving terminal has been reduced to about one-fifth the size of a receiving terminal which is embodied without use of a substrate of high dielectric constant. Still, this means a side length of about 20 to 25 mm which, in applications involving use of a small communications device such as a portable receiving terminal, adds to much volume and weight to the terminal. The above-mentioned PIFA is easier miniaturised, but is devised for linearly-polarised radio waves.
U.S. Pat. No. 6,369,762 to Yanagisawa et al., assigned to Yokowo Co., targets the drawbacks of prior art antennas, particularly pointing out and describing the dominant-mode patch antenna, and proposes an antenna for circularly-polarised waves having a pair of electrodes for radiating a linearly-polarised wave which are provided substantially in parallel with a ground conductor plane, with an excitation electrode interposed there between. A feeding section is electrically connected to the excitation electrode, wherein first ends of the respective radiation electrodes oppose to the excitation electrode, thereby constituting capacitive coupling. Second ends of the respective radiation electrodes are connected to the ground conductor plane such that the directions in which electric fields are to be excited become substantially orthogonal to each other. The structure of the antenna is substantially L shaped, with each of the two orthogonal arms having an electrical length of ¼ of a particular radio wavelength.